Known demodulation systems for such FM sound carriers normally translate the frequency of the selected carrier to a known frequency and the signal is then processed by a single high Q narrow band pass filter. The single carrier frequency is then FM demodulated. Each of the carrier frequencies in the composite signal must be shifted to the same known frequency when it is required to tune onto that selected carrier frequency. To handle stereo signals two such filters are required and as the band pass filter used will have a selected bandwidth, it is necessary to use a plurality of filters to allow for the variation in modulation which may be permitted for different carrier frequencies. The use of such frequency translation to use a common filter presents more problems in providing an economic solution on integrated circuit chips.
Phase locked loops have been used for FM demodulation but have presented problems where a plurality of carrier waves of close frequency have been present. Such phase locked loop circuits have presented problems of stability due to variable gain with process variations particularly on an integrated circuit implementation and they have also been sensitive to variations in the input signal amplitude.
It is an object of the present invention to provide an improved apparatus and method for FM sound carrier demodulation using a phase locked loop.